This special made for Australia (early 1980's model) simply had a ten turn stacked ground balance control. This staked control provides more resolution when setting the ground balance manually.

Regarding the difference between the iron mineralization (ground) in Australia, the iron ground mineral in Australia is typically millions of years older (more decayed) compared to anywhere else in the world. This further decayed iron, is way different compared to iron ground that has not decayed to that level.